Hitherto, there has been known a hot-water supply system having a hot-water supply apparatus of an instantaneous heating type connected in series at a downstream side of a hot-water tank unit in which water in a hot-water tank is heated by a heat pump (for example, refer to Japanese Patent Laid-open No. 2000-329401).
When the hot-water stored in the hot-water tank is being supplied, if the temperature of the hot-water in the hot-water tank has been continued to be low, it is easy for bacteria such as Legionella or the like to reproduce in the hot-water of the hot-water tank. Therefore, there has been proposed a hot-water supply system which boils the hot-water in the hot-water tank up to a high temperature in a time slot of late evening to sterilize the bacteria so as to prevent insanitary hot-water containing the bacteria from being supplied (for example, refer to Patent Document 2: Japanese Patent Laid-open No. 2003-130452).
However, if the time setting of the hot-water supply system is not performed or the setting for boiling the hot-water in the hot-water tank up to a high temperature is not performed, the reproduction of the bacteria in the water of the hot-water tank progresses, which makes it a problem that the water containing a lot of bacteria is supplied from the hot-water supply system.